Vertical cavity surface emitting lasers (VCSEL) are typically formed of a layer stack comprising an active (gain) region, sandwiched between two distributed Bragg reflectors. The layer stack is composed of appropriate dielectric and/or semiconductor layer sequences. The laser cavity is formed of the two DBRs, one of which, for outcoupling purposes, has a lower reflectance at the lasing wavelength than the other one. In case of a vertical extended cavity surface emission laser, the resonating cavity is formed between the DBR having the higher reflectance and an extended mirror. The second DBR which would be arranged inside this extended cavity can be omitted or is designed to have even lower reflectance at the lasing wavelength compared to a VCSEL.
For stable operation of such a VECSEL an optical lens is required inside the extended cavity to form a stable resonator with two flat end mirrors. Furthermore, the heat generated during operation of such a laser must be efficiently dissipated from the active region.
WO 2004/086578 A2 discloses to attach an optically transparent substrate to the free side of the layer stack inside the extended cavity. This transparent substrate is used as a heat spreader and designed to have one or more further selected properties to influence the light which is output from the laser. Examples of such further properties are polarizing properties to achieve a desired polarization of the laser radiation, a nonlinear optical response to achieve frequency doubling of the laser radiation or the provision of a curved surface to focus or defocus the output light.
The thermal conductivity of transparent substrates however is not sufficient to use them as an effective heat sink. Furthermore, a homogeneous current injection into the active region is difficult to achieve when the transparent substrate is not electrically conductive.